1. Field of the Invention
The invention relates generally to radio navigation and more particularly to air-derived guidance systems responsive to a ground based scanning beam transmission format.
2. Description of the Prior Art
Systems are known which air-derive positional information from transmitted ground signals discretely coded in angle by one means or another. Broadly speaking, the well known VOR fits that description. Another system particularly adapted to aircraft landing approach navigation is described in U.S. Pat. No. 3,159,837. U.S. Pat. No. 3,141,166 describes a system in which the angle information in a scanning beam navigational approach beacon is basically time referenced, and it is to this basic type of system that the present invention is most germane.
More recently, a system which transmits a single frequency collimated beam sweeping (scanning) a sector of space and a separate signal of offset frequency directed to illuminate the entire swept sector independently of the scan is described in U.S. patent application Ser. No. 662,342 filed Mar. 1, 1976, now U.S. Pat. No. 4,053,892, and assigned to the assignee of this application. The separate signal of offset frequency is a phase reference in that system, enabling coherent signal processing in the air of a type which was previously known only in two-way systems, such as the familiar coherent MTI radar.
The provision of such a reference signal permits the beat between the two signals (beam and reference) to be detected in the receiver to produce a signal of average frequency which is that of the reference signal. The instantaneous phase of the beat signal at any point in time depends on the phase relationship between the reference signal and the beam signal, and a detected signal which is of a coherent nature is thereby provided. The manner in which the present invention improves on this art will be evident as this description proceeds.